The present invention relates to a semiconductor protection device for protecting a semiconductor switch which generates a surge voltage at the time of current interruption and a power converting system using the same.
As a power electronics system such as a motor driving power converting system, an inverter is known. As a switching element used in the inverter, recently, an IGBT (Insulated Gate Bipolar Transistor) is widely used. In this type of inverter, a protection circuit is used for protecting the IGBT from a surge voltage generated at the time of turn-OFF of the IGBT which is turned ON/OFF at the frequency of 50 Hz to 20 kHz.
As the above type of protection circuit, a clamp type snubber circuit (charging type RCD snubber circuit) shown in FIG. 35 is conventionally used. The clamp type snubber circuit is provided for a series circuit including a filter capacitor Cf for supplying a circuit voltage, a wiring inductance Lm and two IGBTs 1 and 2 which are used as switching elements. More specifically, series circuits including snubber capacitors C1 and C2 and snubber diodes D1 and D2 are respectively connected in parallel with the IGBTs 1 and 2. One-side ends of snubber resistors R1 and R2 are respectively connected to nodes between the capacitors C1 and C2 and the diodes D1 and D2. The other ends of the resistors R1 and R2 are respectively connected to the emitter of the IGBT 2 and the collector of the IGBT 1.
That is, the clamp type snubber circuit has a construction in which the capacitors C1 and C2 are cross-coupled and previously charged to a circuit voltage. At the turn-OFF time, energy stored in the wiring inductance Lm is discharged, and when an instantaneous excessively high voltage (which is hereinafter referred to as a surge voltage) higher than the circuit voltage is applied to the IGBTs 1 and 2, charges are stored (excessively charged) in the capacitors C1 and C2 to clamp the surge voltage. This operation occurs at each turn-OFF time. Since the energy loss corresponds to an excessively charged amount of charges, the clamp type snubber circuit has an advantage that the energy loss is small in comparison with a completely discharging type snubber circuit.
However, the surge voltage suppressing function of a snubber circuit using a capacitor, i.e., either of the completely discharging type snubber circuit and the clamp type snubber circuit, has a disadvantage that the magnitude of the generated surge voltage varies, depending on the magnitude of an interruption current. For example, a surge voltage of 400V is generated when the interruption current is 100 A as shown in FIG. 36 and a surge voltage of 800V is generated when the interruption current is 200 A as shown in FIG. 37. Thus, the surge voltage varies according to the magnitude of the interruption current.
In this case, 100 A corresponds to 100% of a normally used current area of the switching element and 200 A is an excessive current set value such as an accident current and corresponds to 200% of the normal current. In the above application method, a circuit a voltage (1000V)+surge voltage (800V)+marginal amount (200V)=2000V is used as the breakdown voltage of the element. That is, the breakdown voltage of the element becomes approximately 200% of the circuit voltage.
The above relation can be expressed by use of a reverse bias safe operating area (RBSOA) which is necessary for the characteristic of the element, as shown in FIG. 38. That is, it is necessary to safely interrupt the maximum excessive current at the circuit voltage (1000V) and safely interrupt the steady-state current at the maximum voltage (1800V). As is clearly seen from FIG. 38, the safe operation is required in an area higher than the circuit voltage (1000V). However, in the high breakdown voltage area, holes and electrons are easily generated by the avalanche breakdown so as to abruptly make it difficult to attain the safe operation. Therefore, particularly, in order to develop IGBTs which can be used in an area higher than the circuit voltage 1000V, some restrictions for reducing the current used are required, for example.
Further, the clamp type snubber circuit has a cross-coupled wiring and cannot be applied to a circuit in which one arm includes serially connected two or more IGBTS. Therefore, in a high voltage converting system having a plurality of IGBTs serially connected, there is a problem in that a usable snubber circuit of small loss is not realized, but, as a result, a non-charging type snubber of large loss is used.
On the other hand, there is a circuit system having a combination of a snubber circuit and a protection circuit against the excessive voltage, as shown in FIG. 39. The operation of the above circuit is explained with reference to FIGS. 40 and 41.
First, if IGBTs 1 and 2 interrupt a normal current i1 (=100 A at maximum), a surge voltage is generated and it is suppressed to V1 (=1400V at maximum) by a clamp snubber. V1 is suppressed to 150% (=1500V) of the circuit voltage at maximum if the normal current i1 is interrupted. However, if an excessively large current ioc (=200 A at maximum) generated at the time of accident of load short circuit is interrupted, a high surge voltage (1800V) is generated in the clamp snubber circuit. Therefore, it reaches an excessive voltage protection level Vzd (for example, 170% of the circuit voltage=1700V) and is clamped at Vzd, as shown in FIG. 41.
Thus, the element breakdown voltage in a case where the temporary excessive voltage protection circuit of FIG. 39 is attached becomes 1700V+marginal amount 200V=1900V. In this case, an element whose breakdown voltage is lower than the element breakdown voltage 2000V, attained when only the clamp snubber circuit of FIG. 35 is used, can be used, but the difference in the breakdown voltage is small. Since electric energy indicated by the oblique line in the voltage suppressing period is discharged at the turn-ON condition, a current of an amount larger than that of the current flowing at the normal turn-OFF time flows as shown in FIG. 41. Like the case of FIG. 38, the RBSOA of this case is required so that the steady-state current can be safely interrupted at the time of maximum voltage 1700V, and therefore, significant improvement cannot be expected.
Further, an excessive voltage protection circuit different from the excessive voltage protection circuit shown in FIG. 39 is disclosed in Japanese Patent No. 2622524 and Jpn. Pat. Appln. KOKAI Publication No. 7-288456. In the construction disclosed in the above publications, an auxiliary switching element such as an FET or IGBT is connected in parallel with a main switching element and an excessive voltage sensor is provided for the auxiliary switching element. If an excessive voltage is applied, the auxiliary switching element is operated to protect the main switching element from the excessive voltage.
However, the protection method in the above cases is the same as that used in the conventional construction shown in FIGS. 40 and 41 and no improvement can be expected from the viewpoint of utilization factor of the element breakdown voltage. That is, an excessive voltage protection level which is sufficiently higher (for example, 170% as explained in the former case) than the circuit voltage is set and a design is made such that the surge voltage does not exceed the level. Further, the significant improvement cannot be expected from the viewpoint of the RBSOA.
Further, in a case where IGBTs 1 and 2 are connected in parallel instead of using the single IGBT, the characteristics of the IGBTs 1 and 2 are different from each other and the currents flowing therethrough are unbalanced. At this time, a high surge voltage is applied to the snubber circuit by one of the IGBTs which interrupts a larger current. Further, if the IGBTs 1 and 2 are connected in series, surge voltages generated in the IGBTs 1 and 2 at the turn-OFF time become unbalanced and an excessively high surge voltage is applied to one of the IGBTs.
As describe d above, in the conventional snubber circuit, since the surge voltage becomes high in proportion to the magnitude of the in terruption current, the utilization factor of the voltage of the main IGBT is lowered according to the margin for destruction by the excessive voltage. That is, no problem occurs when the interruption current is small as shown in FIG. 36, but when the interruption current is large as shown in FIG. 37, the breakdown voltage is 2000V with respect to the circuit voltage 1000V and the utilization factor is lowered to approx. 50%. Likewise, in a case shown in FIG. 41, the element breakdown voltage is 1900V while the circuit voltage is 1000V and the utilization factor becomes 52.6% and can be enhanced only by 2.6%.
From the viewpoint of the RBSOA, it is required to safely turn OFF the element at a high voltage of 800V or 700V with respect to the circuit voltage 1000V. Further, in the snubber circuit, since the external dimensions of the capacitor and resistor are large, the external dimension of the capacitor becomes extremely large and it becomes difficult to cool the resistor when the breakdown voltage becomes high.
Further, there is a problem that a clamp type snubber circuit of small loss for series connection is not present. Also, there occurs a possibility that surge voltages generated at the time of interruption of the current flowing in the series-connected IGBTs will not be uniformly generated in the respective IGBTs and an excessive voltage is applied to one of the IGBTs to destroy the IGBT.
If the main IGBTs are connected in series or in parallel, the voltages or currents thereof occurring at the switching time become unbalance depending on the characteristics of the main IGBTs. In order to eliminate the unbalanced state, it is required to set the characteristics of the main IGBTs equal. This lowers the yield of the IGBTs and raises the cost thereof.
Further, if the voltages become unbalance at the time of switching of the main IGBT, it becomes difficult to standardize the main IGBT and the peripheral circuit thereof. In this case, it becomes necessary to select the main IGBT and snubber circuit each time the power converting system is formed and the cost thereof may be raised.